Edit: Oh sorry, didn't read carefully. I was thinking of the normal Ubuntu.
Most of the steps should apply though, as Unity is based on GNOME 3.
How To Install XFCE alongside Unity (the enviornment that comes with Ubuntu)
It's recommended to add another user so data doesn't corrupt. (Skip if you don't want to)
Open System Settings (unity-control-center) and click User Accounts. Click the Unlock button on the top right and type in your password as needed. Then, click the + button on the bottom left and select Administrator. Then, choose a Username and Name for the account.
Log out of the account you are in right now and Log In to the account you just made.
Install XFCE
It's recommended to update your sources.
Open Terminal with Ctrl + Alt + T and type
sudo apt-get update
Then, install XFCE!
sudo apt-get install -y xfce4 xfce4-goodies
Log out, click the Ubuntu logo over the account name, and click XFCE.
Another way
You can download Xubuntu from the official website, here
Hope this helped!